The present invention relates to a method of fabricating a semiconductor device and, more particularly, to a method of producing a silicon layer having a surface controlled to be uneven or even. A silicon layer having an uneven surface may be used for an capacitor electrode in a dynamic random access memory (DRAM) cell, and a silicon layer having an even surface may be used for a thin-film-transistor in a static random access memory (SRAM) cell.
Recently, the areas of section available for formation of capacitors are decreasing with the smaller cell sizes due to the increase in the degree of integration of DRAMs. Accordingly, stacked or other type capacitors having larger plate surface areas have been used in order to have satisfactory levels of capacity.
The cell areas, however, must be further reduced as the degree of integration of DRAMs is increased to as high as 64 Mbits and even 256 Mbits, and even with the above-mentioned structures, the insulated plates are required to be ultra-thin. Here, it is to be noted that the thinnest film of SiO.sub.2 /Si.sub.2 N.sub.4 which has become practical is on the order of 4 nm, and thus the above-mentioned problem is difficult to solve by merely thinning the plate films.
A technical solution to this problem is described in Japanese Laid-Open Patent Publication HEI 3-272165. This publication describes that deposition of a silicon film by LPCVD (Low Pressure Chemical Vapor Deposition) involves production of hemispheric silicon grains (hereunder referred to only as "grains") at a temperature around which the crystal state of the film changes from an amorphous to a polycrystalline state. Since application of these grains to electrodes results in provision of irregularities to the electrodes which contribute to much greater amount of accumulated charges, that process is very effective.
The irregularities due to grains, however, can grow only within a certain range of temperatures. There still remains the additional problem of difficult control of grain sizes, etc.
A method as a solution to the problem is described in Japanese Laid-Open Patent Publication HEI 5-304273. This publication discloses a method of growing mushroom-shaped grains by first irradiating the clean surface of an amorphous silicon electrode with disilane gas (Si.sub.2 H.sub.6 gas) to form micro-crystal nuclei thereon, and then attaching, to the micro-crystal nuclei, silicon atoms which migrate on the surface of the amorphous silicon.
This method allows density-controlled, uniform dispersion of grains on the electrode surface. Accordingly, the method is a very effective one to be applied to devices.
In this connection, "Solid State Devices and Materials": issued in 1992 describes, on page 422, that grains with controlled sizes and a density may also be formed on an electrode of phosphorous-doped amorphous silicon, under the title: "Hemispherical Grained Silicon (hereunder abbreviated to "HSG-Si") Formation on in-situ Phosphorous-Doped Amorphous-Si Using the Seeding Method". Accordingly, the above-mentioned method has the advantage of eliminating the need for impurity-doping processing such as ion implantation which causes deformation of the grains, which processing is otherwise required after formation of irregularities on the surface of the amorphous silicon electrode.
In addition, on page 259 of "International Electron Devices Meeting" it is stated that even electrodes having cylindrical structures may have irreguralities thereon which are due to micro-crystals, under the title: "A New Cylindrical Capacitor Using Hemispherical Grained Si (HSG-Si) for 256-Mbit DRAMs". This technology has made possible 256-Mbit DRAMs. For the foregoing reasons, it is understood that the method described in the above publication HEI 5-304273 is a very effective one.
The above-described method, however, has the drawback of failing to form irregularities on amorphous silicon having a phosphorus content of over 5.times.10.sup.20 atoms/cm.sup.3. The reason for this drawback is that the formation of the irregularities is prevented by the phosphorus on the surface of the film, and this will be described in more detail later.
Also, an additional drawback is that the respective grains cannot grow to a satisfactory degree in cases where the sidewall, etc. of the cylindrical electrode is thin, since the formation of irregularities proceeds with supply of atoms from the amorphous silicon.